With the onset of the monsoon season, we've had a few welcome overcast days in the past few weeks, so it's a good time to talk about light.
Indoor hemp growing facilities supplement light for 18-24 hours in order to keep the plants in a vegetative state ("veged"). This allows the hemp plants to grow taller and wider with more branches that yield more flowers in the future. Later, lighting can be manipulated and switched to a 12 on/12 off schedule to initiate flowering. Cannabis is unique in that it needs darkness in order to flower. Sounds simple, right? Well, think again.
Today in Tucson, we will have exactly 13 hours and 56 minutes of daylight. Blazing hot, bright Arizona daylight. Some days the light is so intense that the sky looks white, but it's evident that shorter days have had an effect on hemp plants.
Below is a photograph of a plant in our trial garden that is in full flower mode:
Yet other strains aren't so far along. The photograph below is a hemp plant that's only slightly thinking about flowering:
So what gives? Well, just as different strains have different water and nutrient needs, they also have different light needs.
Lighting for crops is typically measured in micromoles. One micromole is equal to one millionth of a mole, totaling the amount of photons that are being delivered to the crop per square meter per second. The measurement is a function of light intensity over time. But it's not the amount of micromoles at any given moment that's important-- what's important is the total micromoles obtained throughout the photo period.
Think of it in terms of daily caloric intake for humans. The typical human needs about 2000 calories per day. That human can either obtain the calories from a single supersized Big Mac meal at McDonald's or by eating fruits and vegetables throughout the entire day. Now apply that anecdote to hemp and light-- as long as the hemp plant obtains a minimum number of micromoles during that photo period, it will stay in a vegetative state.
As summer progresses and the days begin to get shorter, we will continue to learn more about how hemp performs in the Southwest. By measuring light daily, we learn more about each hemp strain in relation to sunlight at our latitude, and we will select strains that perform the best in our climate.